NullPointerException

Hello everybody,I have an application where i use TDA as Map and so I have a Map of Maps where I store keys and values as String and Double as you see from the code bellow. So a Map that stores other Maps where I store information of movies and ratings. Then in the Map that contains the other Maps i store as value the name of the person that have given this oppinion about movies . I have implemented a method that calculates the euclidean distance between two users and it functions but when I try to use the same method inside another method that calculates the same thin but between a user and all the others it gives me an NullPointerExeption despite it compiles well.Can somebody give me an advice?